A secondary battery quick-charging circuit is disclosed in Japanese Laid-Open Utility Model Application No. 134231/1979. According to this application, a silicon controlled rectifier is controlled by the charging current of a capacitor. This arrangement is disadvantageous because in the event the supply voltage varies after the battery has been fully charged and during a period when overcharge is being prevented, a charging current flows in the capacitor and the silicon controlled rectifier is erroneously operated.
Japanese Laid-Open Utility Model Application No. 36745/1982 discloses a secondary battery quick-charging circuit wherein after the battery has been fully charged, a small current continues to flow in a transistor. The disdvantage of this arrangement is the resultant loss of electric power irrespective of the battery charging operation.
In view of the foregoing disadvantages, U.S. Pat. No. 4,623,832 or the corresponding Japanse LaidOpen Utility Model Application No. 75728/1984 discloses an improved secondary battery quick-charging circuit wherein the secondary battery is quickly charged by a large current. The fact that the batter has been fully charged is detected, and the circuit is arranged so that a malfunction does not occur caused by the voltage variation in a voltage source at the time of charging with the small current. This arrangement is still disadvantageous in that a thermostat generates heat attendant to the increase in the charging current and the thermostat is rendered open before the battery has fully charged. This is due to the fact that the circuit includes in a charging path a series circuit comprising the secondary battery, the thermostat and the silicon controller rectifier, and that a current capacity of the thermostat is small.